Add highlight for instance group dock (#1860)

* Add connect callback to instance group

* Add QtInstance instead of InstanceGroup

* Add changes for review

* Handle case when select is neither Instance, int, nor InstanceGroup

---------

Co-authored-by: roomrys <38435167+roomrys@users.noreply.github.com>

sleap/gui/widgets/video.py

@@ -74,6 +74,7 @@
 from sleap.prefs import prefs
 from sleap.skeleton import Node
 from sleap.io.cameras import Camcorder
+from sleap.io.cameras import InstanceGroup
 
 
 class LoadImageWorker(QtCore.QObject):
@@ -272,6 +273,7 @@ def update_selection_state(a, b):
         self.state.connect("frame_idx", lambda idx: self.plot())
         self.state.connect("frame_idx", lambda idx: self.seekbar.setValue(idx))
         self.state.connect("instance", self.view.selectInstance)
+        self.state.connect("instance_group", self.view.selectInstance)
 
         self.state.connect("show instances", self.plot)
         self.state.connect("show labels", self.plot)
@@ -960,18 +962,42 @@ def all_instances(self) -> List["QtInstance"]:
         scene_items = self.scene.items(Qt.SortOrder.AscendingOrder)
         return list(filter(lambda x: isinstance(x, QtInstance), scene_items))
 
-    def selectInstance(self, select: Union[Instance, int]):
-        """
-        Select a particular instance in view.
+    def selectInstance(self, select: Optional[Union[Instance, int, InstanceGroup]]):
+        """Select a particular instance in view.
 
         Args:
-            select: Either `Instance` or index of instance in view.
+            select: Either `None` or `Instance`, index, or `InstanceGroup` of instance
+                in view.
 
         Returns:
             None
         """
+
+        # Decide which function to use to determine if instance is selected
+        if isinstance(select, int):
+
+            def determine_selected(idx: int, instance: QtInstance):
+                return idx == select
+
+        elif isinstance(select, Instance):
+
+            def determine_selected(idx: int, instance: QtInstance):
+                return instance.instance == select
+
+        elif isinstance(select, InstanceGroup):
+
+            def determine_selected(idx: int, instance: QtInstance):
+                return instance.instance in select.instances
+
+        else:
+
+            def determine_selected(idx: int, instance: QtInstance):
+                return False
+
+        # Set selected state for each instance
         for idx, instance in enumerate(self.all_instances):
-            instance.selected = select == idx or select == instance.instance
+            instance.selected = determine_selected(idx, instance)
+
         self.updatedSelection.emit()
 
     def getSelectionIndex(self) -> Optional[int]: